1. Field of the Invention
The present invention relates to an image pickup optical system, and more particularly to a low-cost optical lens assembly that is comprised of five lens elements that constitute the total length of the optical lens assembly and applied to an electronic product.
2. Description of the Related Art
As science and technology advance, the development of electronic products such as mini or thin digital still cameras, web cameras or mobile phone cameras tends to have a compact design to meet the user requirements for an optical lens assembly with good aberration correction ability, high resolution, and high image quality.
In general, a conventional optical lens assembly of a mini electronic product comes with different designs, including the two-lens, three-lens, four-lens, and five-or-more lens designs. If the image quality is taken into consideration, the optical lens assemblies with the four-lens and five-lens designs have advantages on aberration correction and modulation transfer function (MTF), and the five-lens design having a higher resolution than the four-lens design is applicable for electronic products of high video quality or high pixel requirement.
In various mini five-lens designs of an image pickup optical system with a fixed focal length, different combinations of positive and negative refractive powers are adopted. As disclosed in Japanese Pat. Publication Nos. JP2003-131136 and JP2005-266771, two sets of cemented doublets are used for reducing the total length of the optical system. As disclosed in Japanese Pat. Publication Nos. JP2003-185917 and JP2006-293042, U.S. Pat. Publication Nos. US2004/0196571 and US2003/0117722, and R.O.C. Pat. No. TW M313781, one set of cemented doublet is used to achieve a wide-angle effect. As disclosed in Japanese Pat. Publication No. JP2003-161879, a fourth-lens and a fifth-lens with different refractive powers are used to construct an optical system, but the total length of such optical system is not applicable for compact electronic devices.
In small products such as a digital camera, web camera, and mobile phone camera, the optical lens assembly requires a compact design, a short focal length and a good aberration correction. For different designs of the image pickup optical systems with a fixed focal length and five lens elements, a better aberration correction and an appropriate total length can be achieved by designing the fourth and fifth lens elements with different refractive powers and an inflection point. As disclosed in R.O.C. Pat. Nos. TWM313246, TW201038966, TW201022714 and TWM332199 and U.S. Pat. No. 7,710,665, a good aberration correction is achieved, but the total length of the optical system still cannot meet the requirements of compact electronic devices. As disclosed in U.S. Pat. No. 7,826,151, US2010/0254029 and US2010/0253829, the fourth lens element and the fifth lens element having an inflection point are adopted for the design of the optical system with a shorter total length, but these patents use the fourth lens element and the fifth lens element having the inflection point to correct the aberration or image distortion, and thus a longer air gap exists between the third lens element and the fourth lens element which is not desirable for a shorter total length design. Therefore, the present invention provides a feasible design to shorten the optical lens assembly, while using the refractive power of the five lens elements and the combination of convex and concave surfaces to improve the image quality for the application on compact electronic devices, in addition to the effect of reducing the total length of the optical lens assembly.